1. Technical Field
The present invention relates in general to digital data transmission systems, and more particularly to an intelligent multistation access unit having a transmission speed detection circuit for a multiple speed data transmission system.
2. Description of the Related Art
In digital data transmission systems, composite clock and data signals in binary form are transmitted over media such as wires or fiber optic cables from a transmission line transmitter to a transmission line receiver. The transmitter and the receiver in a data transmission system may each be a single computer or may each comprise a local area network (LAN) of computers.
An individual computer or station in a LAN may both send information to other stations in the LAN and receive information from other stations. The station inserts into the LAN when it desires to communicate with another station in the LAN, and detaches from the LAN when the communications are complete. If information received by a station is destined for a station further along the network, the receiving station must pass the information along the LAN to the next adjacent station, and so forth, until the information reaches its final destination.
The signal received by a particular station in the LAN is represented by pulses in a data stream defined by positive-going and negative-going transitions. The transmitting station in the LAN outputs the data signal at a predetermined frequency. The binary data waveform, however, is degraded with respect to its phase and frequency as it propagates along the LAN transmission media due to electrical noise and dispersion. This degradation of bits in the binary waveform could result in incorrect interpretations by the receiving station of bits sent by the transmitter station across the LAN transmission media.
To prevent incorrect interpretation of data bits sent across stations in the LAN, the receiving station must reconstruct the received signal, regardless of electrical noise and transmission media degradations. Such reconstruction can be achieved by sampling the received signal at a regular rate equal to the transmitted bit rate, and at each sample instant making a decision of the most probable symbol being transmitted. Typically, a threshold level is chosen to which the received signal is compared. Transitions above and below this level are deemed to be binary ones or zeros, depending on the type of encoding of the signal.
In addition to reconstructing received data signals prior to retransmission to adjacent stations in the LAN, any new information which the station outputs to the LAN must be transmitted at the same frequency as that at which the LAN is operating. Because some LANs are capable of operating at different speeds, individual computers or stations in the LAN must be matched to the speed at which the LAN will be operating. Typically, the individual stations are programmed to the speed at which the LAN is to be operated. If the speed of the LAN is to change, the LAN must be taken down, and all the stations must be reprogrammed to the new speed at which the LAN will be operating. If a particular station is programmed to a speed of operation which is different than the speed at which the LAN is operating, any attempt by the station to attach to the LAN will result in bringing the LAN down.
A common LAN topology is the "token ring" network. The token ring is used to interconnect the devices attached to the network. The token ring network allows unidirectional data transmission between stations in a ring-like circuit, by a token passing procedure. The ring topology permits tokens to be passed from a node associated with a particular attached device, such as a personal computer, to another node in the ring. A node that is ready to send data can capture the token and thereafter insert data for transmission. A device or computer station attempting to gain access to a node of the token ring will have a token adapter which is physically connected to the token ring. This accessing device must carry out a procedure following a standard protocol in order to access the token ring.
Many LANs employ concentrators, or hubs, to connect many stations at a single network node. These multistation access units connect individually with each station along a 4-wire cable called a lobe. Multiple lobes extend out from a concentrator to individual stations to form a star-like structure. Physically, each station is individually attached to the concentrator through its lobe, where it may access the network node. When the concentrator is connected to a token ring network, the logical configuration of the network places each station connected to the concentrator at a separate node within the ring. A concentrator can individually connect the attached devices in a token ring, or it may be connected with other concentrators to form a larger token ring comprised of all the devices attached to all concentrators. An intelligent concentrator is one which includes processor controlled switching electronics for controlling access to the network.
One type of token ring product has two data transmission speeds, 4 Mbps and 16 Mbps. Both of the transfer speeds are frequently used, and often, the data transmission speed of 4 Mbps may be used in one network, while the data transmission speed of 16 Mbps may be used in another network, both of which a user may wish to access.
The speed of all devices inserted in the token ring must be the same for the ring to operate properly. Unfortunately, the end user of the accessing device may have no way of knowing the ring speed of the particular token ring he is attempting to access. If this user tries to insert onto the ring at the wrong speed, his station will cause the entire ring to go into a beacon state in which all data transmission is halted until the station is removed from the ring. This time can range from several milliseconds to 20 seconds depending on the wiring concentrator used. In addition, some non-token ring devices can be accidentally connected to a network node to cause complete failure of the ring until that device is found and manually removed.
The prior art has solved this problem of accessing multiple transmission rate networks by providing network adapters, for placement in computer workstations, which are able to detect the transmission speed of the ring and then configure the adapter to transmit at the proper rate. However, in order for the adapter's speed detect circuitry to determine the ring speed, the adapter must insert itself on the ring and sample data transmissions. Such speed detection inherently interferes with the data transmission on the ring, and further, may send the entire ring into a beacon state. Such disruptions in the data communication reduces network performance. Also, such schemes require complex clock extraction and data regeneration logic to implement. Moreover, these prior art schemes deny the token ring any facility for preventing insertion of a non-token ring device into the network, which will cause complete failure of the ring.
It would therefore be desirable to provide a system and method for inserting a device into a multiple transmission rate digital data communications network at the proper transmission speed without disrupting data flow in the network. It would be further desirable to provide a system and method of preventing non-network devices from inserting into the ring and disrupting data flow.